Summary notes of the CMS solenoid stray field on 13/01/2009
Present: BD, MG, RdM, FS, RT, YS, FZ (part time)
General introduction
The background information was reported in LHC Performance Committee meetings and distributed to SF:
First presentation by Mirko Pojer (ppt): measurements on the right side of IR5. No information available on the direction of the field.
Additional measurements by Boris Bellesia (ppt): a directional probe was used. However, the information concerning the sign of the various component is missing. A discussion with Mirko and Boris led to the conclusion that these measurements are really hard to exploit.
Last measurement by Mirko Pojer (ppt): a directional probe was used to measure the field on the left side of IR5. The reference system, although with a non-conventional naming of the axis, is coherent with the naming of the magnetic-field components. The field-components feature change of signs. A discussion with Mirko highlighted that the measurements are very difficult to be performed due to the limited space available, which makes it difficult to ensure a correct alignment of the probe. This affects the accuracy of the measured values, in particular the transverse field components. It seems reasonable to assume that values below 2 mT are not really meaningful.
A meeting with S. Russenschuck was organized (YS, FZ, and MG): the longitudinal field should become zero at the entrance of the Q1. The cryostat will act as an effective magnetic shield for the stray field. However, at the level of the interconnects there might be field leakage in the beam pipe (SF and confirmed by SR). The order of magnitude of this effect is difficult to estimate. SR will launch numerical simulations to evaluate these effects.
Discussion
BD presented the study performed in the framework of the CLIC activities (pdf). the measured field map of the CMS solenoid is interpolated. The field should be used to perform beam dynamics simulations. The relative direction between the beam and the solenoid axis has to be taken into account for the CLIC case (crossing angle of 10 mrad).
From the ensuing discussion, the following actions were decided:
YS will interpolate the measured field map of the CMS solenoid (provided by BD).
The extension of the longitudinal field-component will be stretched up to the entrance of the Q1 quadrupole. The field level in front of the Q1 will be derived from the magnetic measurements.
A series of thin solenoids will be introduced to simulate the effect of the measured stray field (assumed to be a pure solenoid). MAD-X is already in good shape to perform these type of simulations. For the time being, the potential field leakage through the triplets interconnects is neglected.
The SixTrack convertor will have to be upgraded to allow solenoids to be used in SixTrack (to be done by FS and YS).
This strategy will allow to estimate the linear effect of the solenoid including the stray field. A correct, though approximate, way to take into account the radial dependence of the longitudinal field in the beam dynamics should be defined.
The next meeting will be in two weeks.
Last update: 13-January-09
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